debian: package mgr/rgw in ceph-mgr-modules-core

debian: package mgr/rgw in ceph-mgr-modules-core

in 110db72e, we added the rgw mgr module to ceph-mgr-modules-core
rpm package. but we didn't add this module to the corresponding
debian package.

rgw mgr module provides a simple interface to deploy RGW multisite
setup. so it would be nice to have it in ceph's debian packages as
well.

despite that rgw is not part of the core features, since this module
is already in ceph-mgr-modules-core rpm package, and it is relatively
small and does not pulling extra dependencies, let's added to the
debian packge with the same name. we can revisit this decision and
extract it out in a following up change if it is necessary in future.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>